Directional microphones typically are divided into two groups: first order and second order set-ups. In a first order set-up (see FIGS. 1 and 2), sound from two spatially different inputs is picked up and processed. To obtain directionality, the sound of a first inlet is delayed after which the two input signals are subtracted. This so-called delay-and-subtract process can be performed by a processing circuit in a two microphone setup as shown in FIG. 1 or by a mechanical equivalently configured single microphone setup as shown in FIG. 2. Naturally, these set-up types may be combined, as may be seen in FIG. 3, where two directional microphones are used in a second order set-up. In the second order set-up (see FIG. 3), two microphones each pick up sound from two spatially different inputs, and the delay-and-subtract process is performed twice, once mechanical and once in circuitry. In addition, the pair of two spatially different inputs of one microphone is usually spatially different from the pair of inputs of the other microphone.
Many of today's directional microphone hearing aids utilize a two microphone approach, using two omni-directional microphones in end-fire geometry. A first-order delay-and-subtract processing creates a spatial dependent sensitivity with the maximum located directly in front. This spatially dependent sensitivity (“directionality”) has proven to be beneficial for speech intelligibility in noisy environments.
A drawback of using the delay-and-subtract processing is that the sensitivity of the microphone array drops with 6 dB/oct at the low frequencies. This makes that a hearing aid utilizing two (omni-) microphone array has worse signal-to-noise ratio than that with a single microphone.
To improve the directionality of hearing aids even further and hence the speech intelligibility, hearing aid manufacturers have been working on utilizing the same delay-and-subtract processing, but now with two conventional, single-cartridge directional microphones (FIG. 3), thus constituting a second order directional set-up.
The sensitivity of single-cartridge directional microphones however drops also with 6 dB/oct for the low frequencies. This, together with the delay-and-subtract processing, makes the sensitivity of the array decrease very rapidly with 12 dB/oct for the low frequencies. As such, second-order directional microphone arrays have a very poor signal-to-noise ratio.
The very low signal(-to-noise ratio) of second-order directional microphone arrays has a negative side effect. It makes the array extremely sensitive to external noise sources, like wind noise or mechanical vibrations. These external noise sources can ‘easily’ deteriorate the directionality and/or cause loud annoying sounds.
This is why hearing aids are rarely equipped with a second-order directional mode. And if used, the working range of a second-order directional mode is limited to the high frequency range only, i.e. ca>2 kHz.
As such, it is desirable to have a second-order directional microphone array with improved signal-to-noise ratio as well as one which is less susceptible to mechanical vibrations. Also, it is desirable to provide a microphone which is less sensitive to vibration etc.